Swift XRT Observations of the Afterglow of XRF 050416A

• Published in: The Astrophysical journal Vol. 654; no. 1; pp. 403 - 412
• Main Authors: Mangano, Vanessa, La Parola, Valentina, Cusumano, Giancarlo, Mineo, Teresa, Malesani, Daniele, Dyks, Jaroslaw, Campana, Sergio, Capalbi, Milvia, Chincarini, Guido, Giommi, Paolo, Moretti, Alberto, Perri, Matteo, Romano, Patrizia, Tagliaferri, Gianpiero, Burrows, David N, Gehrels, Neil, Godet, Olivier, Holland, Stephen T, Kennea, Jamie A, Page, Kim L, Racusin, Judith L, Roming, Peter W. A, Zhang, Bing
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 01.01.2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Afterglow; Gamma ray burst; Cosmic gamma sources; X-rays: individual (XRF 050416A); gamma rays: bursts
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/509100

Swift discovered XRF 050416A with the Burst Alert Telescope and began observing it with its narrow-field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast (decay slope a 6 2.4), subsequently flattens (a 6 0.44), and eventually steepens again (a 6 0.88), similar to many X-ray afterglows. The first and second phases end 6172 and 61450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index 6 3.0 during the initial steep decay, to a harder emission with 6 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of 66.8 x 10 super(21) cm super(-2). The consistency of the initial photon index with the high-energy BAT photon index suggests that the initial fast decaying phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than a few tens of degrees, although numerical simulations show that the late-time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored.